1. Field of the Invention
The present invention relates to the field of Micro Electromechanical system (MEMS) devices. It relates to electrostatic MEMS devices as variable capacitor devices and a method for producing them.
2. Description of the Related Technology
Different electrostatic actuators are known.
Comb-drive structures as an electrostatic actuator are known [Sterken T. et al., An electret-based electrostatic micro-generator, Proceedings of Transducers, The 12th International Conference on Solid State Sensors, Actuators and Microsystems, 8-12 Jun. 2003, Boston, USA, IEEE 2003, p. 1291-1294.]. This type of devices requires thick films or armatures and is therefore often realized by bulk processing, i.e. patterning the bulk of the substrate and using itself as a moving part of the device. Upon application of a bias voltage between the electrodes (fingers), the sets of inter-digitated fingers, shown e.g. in FIG. 1, shift into each other. The movement uses the single degree of freedom available to the system, i.e. the direction of the fingers; in other words the devices move in-plane.
Parallel-plate capacitors as an electrostatic actuator are also known [X. Rottenberg et al., RF-MEMS technology platform for agile mobile and satellite communications, Proc. of the 36th European Microwave Conf., pp. 1723-1726, September 2006]. This type of devices is often preferred for on-chip actuators as they only require in principle a few additional thin-film processing steps. This type of structures is indeed produced by a typical 4-mask process flow realized on top of a dielectric (insulated) substrate. Upon actuation, the movable plate or electrode gets nearer to the fixed, bottom electrode one; the device moves out-of-plane as shown e.g. in FIG. 2, whereby the movable electrode approaches the bottom electrode. The electrodes remain in different planes.
Examples of electrostatic devices (sensors and actuators) are RF-MEMS switches, switchable capacitors, tilting mirrors, power scavengers, accelerometers . . . etc. These prior art devices have the problem that integration of a 4-masks process in/or above another process is rather difficult. Moreover the dielectric layer is typically sensitive to various charging mechanisms. The release process also has a strong impact on the dielectric properties. Often malfunction of clamped-free devices occurs due to the stress-gradient in the armature.
A three-dimensional variable capacitor is described in US-patent 2005/6906905. The electrodes of the capacitor are formed in an interdigitated manner with movable MEMS comb drive electrode beams. Control or actuation electrodes on the beams drive their motion either individually or all in unison, leading to changes in the capacitance. The structure of this variable capacitor is very complex.
It is desirable to provide a variable capacitor which shows a less complex structure and which is as a result easier to manufacture.
It is further desirable to provide a simple method for producing a variable capacitor.